U.S. patent application number 09/819279 was filed with the patent office on 2001-10-25 for light modulation remover.
Invention is credited to Bakker, Johannes Nicolaas, De Haan, Gerard, Kettenis, Jeroen Maria, Mevissen, Perry Gerard, Ojo, Olukayode Anthony, Van Rooy, Johannes Henricus Jozef Maria.
Application Number | 20010033334 09/819279 |
Document ID | / |
Family ID | 8168284 |
Filed Date | 2001-10-25 |
United States Patent
Application |
20010033334 |
Kind Code |
A1 |
Bakker, Johannes Nicolaas ;
et al. |
October 25, 2001 |
Light modulation remover
Abstract
The quality of recorded images with for example a high speed
camera can be detonated by light modulation which is a consequence
of for example gas-discharge lamps running at a lower frequency
with respect to the camera. A camera according to the invention
provides light modulation removal means which tackle this problem
by handling fields of the image different depending on the light
modulation.
Inventors: |
Bakker, Johannes Nicolaas;
(Eindhoven, NL) ; De Haan, Gerard; (Eindhoven,
NL) ; Kettenis, Jeroen Maria; (Hamburg, DE) ;
Mevissen, Perry Gerard; (Eindhoven, NL) ; Ojo,
Olukayode Anthony; (Eindhoven, NL) ; Van Rooy,
Johannes Henricus Jozef Maria; (Breda, NL) |
Correspondence
Address: |
Michael E. Marion
U.S. Philips Corporation
580 White Plains Road
Tarrytown
NY
10591
US
|
Family ID: |
8168284 |
Appl. No.: |
09/819279 |
Filed: |
March 28, 2001 |
Current U.S.
Class: |
348/226.1 ;
348/241; 348/370; 348/E5.034; 348/E5.041 |
Current CPC
Class: |
H04N 5/235 20130101;
H04N 5/243 20130101 |
Class at
Publication: |
348/226 ;
348/370; 348/241 |
International
Class: |
H04N 005/217 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 30, 2000 |
EP |
00106807.1 |
Claims
1. Camera for recording pictures comprising an image sensor for
receiving a picture, a processing unit for processing the picture
and an end processing unit, characterized in that the camera
comprises a light modulation removal means between the processing
unit and the end processing unit for removing light modulation
between different fields of the picture.
2. Camera as claimed in claim 1, characterized in that the light
modulation removal means comprise adaptive fading means for fading
between one field and at least n fields, whereby n is the
repetition pattern of light modulation.
3. Camera as claimed in claim 2, characterized in that the light
modulation removal means comprise means to calculate the lowest
common multiple of the repetition period of said illumination
variation and the repetition period of said picture, which lowest
common multiple is used as common period to average consecutive
images of said picture during recording.
4. Camera as claimed in claim 3, characterized in that the light
modulation removal means comprise a motion detector and means to
decrease the averaging of consecutive images when motion is
detected, which motion detector comprises evaluation means to
evaluate the local difference between images having a field
difference of n.
5. Camera as claimed in claim 3, characterized in that the light
modulation removal means comprises means to estimate the modulation
strength on a locality of the image, and reducing means to reduce
the averaging on localities where the light modulation is weak.
6. Camera as claimed in claim 3, characterized in that the light
modulation means comprises means to reduce the averaging on
localities where the luminance component of said picture is
low.
7. Camera as claimed in claim 3, characterized in that means to
exclude high spatial frequency components of the picture from the
averaging step.
8. Camera as claimed in claim 3, characterized in that the light
modulation removal means comprise means to correct consecutive
images to the same temporal position using motion compensated
conversion techniques prior to the averaging.
9. Camera as claimed in claim 1, characterized in that the light
modulation removal means comprise de-interlacing means to generated
information fro any missing position in the original interlaced
image, using two images with different interlace phases and equal
light modulation phases.
10. Light modulation removal means for use in a camera system
according to claim 1.
11. Method of removing light modulation during recording pictures
with an image sensor having the step of receiving the picture,
processing the picture, removing the light modulation by storing
different field of the picture and averaging the different fields
in dependence of motion, and/or locations with low respectively
high luminance locations.
Description
[0001] The invention relates to a camera as described in the
preamble of claim 1.
[0002] The invention further relates to a method of recording an
image.
[0003] These cameras are known and are used for example to
broadcast sport events etcetera. In such a case a camera can be
used that runs at for example three times the normal picture rate
(150/180 Hz). When such camera has to operate for example under
artificial light, although in general much effort is paid to good
light conditions, the camera can suffer from severe unwanted light
modulation effects working under for example gas-discharge
lamps.
[0004] A disadvantage of a camera working under such light
conditions is that the 50/60 Hz beat frequency between the 150/180
Hz camera and the rectified-mains components in the light source
(100/120 Hz) generates an unwanted modulation of the video signal.
Even though the beat frequency itself is already visible, the
recorded video is supposed to be viewed at normal speed, which down
converts the beat frequency by a factor 3 (when the camera runs at
three times the normal picture rate). When the camera has to
operate under gas-discharge lamps the modulation can get even
worse. To improve this situation it is proposed to spread the lamps
over all the three available main-faces, but this still does not
solve the problem. This is caused, for example because some objects
are less illuminated or due to some reason reflect only one or two
light faces. In consequence, a complex unwanted light modulation of
the images is the result. An additional problem is the light
changing color temperature during the cyclic discharge of the
lamp.
[0005] It is inter alia an object of the invention to provide a
camera and a method, which does not have the disadvantages of the
prior art camera. It is further an object to provide light
modulation removal means for use in such a camera system.
[0006] This is achieved according to the invention by a camera as
described in claim 1. The solution is proposed by the camera
according to the invention, which is based on the processing of the
images during the least common multiple of the camera acquisition
and the rectified mains-period. This beat frequency period
separates images having the same light modulation, the only
difference here are the effects of movement on the scene. An
embodiment of the invention is described in the dependent
Claims.
[0007] These and other objects of the invention will be elucidated
with reference to the Figures.
[0008] FIG. 1 shows a schematic embodiment of a camera according to
the invention,
[0009] FIG. 2 shows an embodiment of a processing unit according to
the invention.
[0010] FIG. 1 shows a schematic embodiment of a camera CM1
according to the invention. The camera comprises an image sensor
IS1 which supplies RGB signals to a sensor unit SU1. The sensor
unit is coupled to a processing unit PU1 for processing the
received RGB signals and supplying YCrCb signals to a light
modulation removal unit LMRU1 which is coupled to an end processing
unit EPU1. The output of the end processing unit supplies the
signals which are known by the man skilled in the art and for
example are coupled to a recording device, etc.. The light
modulation removal unit LMRU1 will be described in more detail with
reference to FIG. 2 hereafter.
[0011] FIG. 2 shows in more detail block schematic a light
modulation removal unit LMRU2 according to the invention. At a
video input VI the light modulation removal unit receives the YCrCb
signals from the processing unit PU2 (see FIG. 1). The light
modulation removal unit LMRU2 comprises different parts. A first
part having in this example five field memories FM1-FM5 for storing
different fields of the received signal. A second part comprising
of (in this example) three de-interlacers DIL1-DIL3. Further a part
having three motion detectors MD1-MD3. Further a part having a
modulation estimator ME and a part having an averager A. After the
different fields are stored in the field memories FM1-FM5 the
fields are supplied to three de-interlacers whereby the first
de-interlacer DIL1 receives at a first input the input signals
after filtered in a band split filter BF1 and at a second input the
output signals of FM3. The second de-interlacer DIL2 receives at a
first input the output signals of FM1 after filtering in a second
band split filter BF2 and at a second input receives the output
signal of a field memory FM4. The third de-interlacer DIL3 receives
at the first input signals at output of field memory FM2 and at the
second input the output signals after field memory FM5. The output
of the de-interlacers DIL1-DIL3 is supplied as a first input to
respectively motion detector MD1-MD3. At the second input the
motion detector MD1 receives the output signal of field memory FM3.
At a second input the motion detector MD2 receives the output
signal of FM4 and the second input of motion detector MD3 receives
the output signal of field memory FM5. The outputs of the motion
detectors MD1-MD3 are supplied to a maximizer MX which supplies at
the output the maximum of the three input signals. This output
signal is supplied via an amplifier AMP to a subtractor at the
negative input. The amplifier AMP receives at a control input a
motion sensitive signal MSS.
[0012] The modulation estimator ME receives at a first input the
output signal of the de-interlacer DIL1, at a second input the
output signal of the band split filter BF2 and at the third input
the output signal of the de-interlacer DIL3. The output of the
modulation estimator ME is coupled via a unit LUT2 to an amplifier
AMP2. At the control input the amplifier AMP2 receives the output
signal of the subtractor SUB. The output of the amplifier AMP2 is
coupled via a third amplifier AMP3 to a fader unit FU. The output
of the fader unit is coupled via a summing unit SUM to the output
VO of the light modulation removal unit LMRU2.
[0013] The averager A receives at the first input the output signal
of the de-interlacer DIL1 at the second input the output signal of
the band split filter BF2 and at the third input the output of the
de-interlacer DIL3. The averager gives at the output the average of
these three input signals to the fader unit FU and also via a
low-pass filter LPF and a unit LUT1 to the amplifier AMP3 as a
control signal.
[0014] In this way it is possible to remove the light modulations
of the received input signals. The solution is based on the
processing of the images during the smallest common product of the
camera acquisition and the rectified-mains period. This beat
frequency period separates images having the same light modulation;
the only differences here are the effect of movement on the scene.
Further, for stationary scenes, an average of the images during the
beat frequency period gives a full removal of light modulation as a
result.
[0015] As temporal averaging of pictures result in movement blur, a
movement detector is used to fade the output between averages and
original video. The movement detection is based on differences
between fields with the same light modulation. A low-pass filter,
to reduce decision noise, filters the max of differences of all
light phases. Before feeding the motion control to the fader, a
linear gain realizes a sensitiveness adjustment. The beat frequency
is assumed equal to the standard field repetition divided by the
highest common factor of field frequency and light frequency e.g.
150 Hz camera with light frequency 100 Hz (2x the main
frequency)=150/HCF(150,100)=3 different illumination phases for
acquisition. Having a 50 Hz camera with light frequency of 120 HZ,
this results in 50/HCF(120,50)=5 fields.
[0016] In case the original images are available in interlaced
format, consecutive picture can not just be averaged together
without a serious reduction in vertical resolution of the
respective video material. Using consecutive images of an
interlaced video stream to extract image features like motion and
others is also very difficult in images areas containing high
vertical spatial frequencies. The reason for those problems is that
two consecutive fields do not represent the same spatial position
of the image. Therefore de-interlacing techniques have been used to
restore the complete frame at each temporal position of the
original image fields. In this way all spatial positions of the
images are available for any processing at any needed input field
time. To prevent the unwanted modulation, present on the original
video images, of disturbing the de-interlacing process, the inputs
are taken with a temporal distance equal to the said common period,
in this way no modulation differences between both images is
expected to be present.
[0017] Looking to stationary pictures, the quality of the averaged
output depends on the used de-interlacer. In the case of a
progressive input, the de-interlacer becomes redundant. Here
averaging gives also a wanted noise reduction effect. For moving
objects in the picture, the light modulation is still present.
Although, in general, this concerns only small portions of the
total disturbances, depending on the scene, their presence is still
annoying. Up on that, the visibility of this residual light
modulation is emphasized by the absence of the removed overall
disturbances.
[0018] Moving areas with small details, where motion detection
fails, are also averaged, leading to detail loss (e.g. grass during
camera panning). To avoid this, estimation is made of the local
strength of the light modulation. If no modulation is measured at
the input of the average function the fader is set towards the
original video.
[0019] To further reduce artifacts the averaging action is also
made inversely proportional to the local luminance value. This can
be introduced in the system due to the light flicker sensitivity of
the human eye. Tempering the averaging action where the light
modulation is already less visible.
[0020] A big improvement can be reached by applying motion
compensation techniques, interpolating pictures from different
light phases to the same time moment before averaging them
together.
* * * * *